The role of leaf hydraulic conductance dynamics on the timing of leaf senescence.

We tested the hypothesis that an age-dependent reduction in leaf hydraulic conductance (Kleaf) influences the timing of leaf senescence via limitation of the stomatal aperture on xylem compound delivery to leaves of tomato (Solanum lycopersicum L.), the tropical trees Anacardium excelsum Kunth, Pittoniotis trichantha Griseb, and the temperate trees Acer saccharum Marsh. and Quercus rubra L. The onset of leaf senescence was preceded by a decline in Kleaf in tomato and the tropical trees, but not in the temperate trees. Age-dependent changes in Kleaf in tomato were driven by a reduction in leaf vein density without a proportional increase in the xylem hydraulic supply. A decline in stomatal conductance accompanied Kleaf reduction with age in tomato but not in tropical and temperate tree species. Experimental manipulations that reduce the flow of xylem-transported compounds into leaves with open stomata induced early leaf senescence in tomato and A. excelsum, but not in P. trichantha, A. saccharum and Q. rubra leaves. We propose that in tomato, a reduction in Kleaf limits the delivery of xylem-transported compounds into the leaves, thus making them vulnerable to senescence. In the tropical evergreen tree A. excelsum, xylem-transported compounds may play a role in signalling the timing of senescence but are not under leaf hydraulic regulation; leaf senescence in the deciduous trees A. trichanta, A. saccharum and Q. rubra is not influenced by leaf vascular transport.